brackets denote choosing between a number of characters in regex. in this case, picking one of "a", "e", "i", "o", "u", "A", "E", "I", "O", or "U".

I appreciate that you throroughly tested your solution.

CodeWars is a relatively safe environment for running code, obviously, but relying on an eval/exec call (especially if you're leaving it bare to the user of a function) should always evoke a deep, finger-curling pain.

It's important to note here that True in Python is basically 1, thus when you add two Trues it's coerced to 2, which allows this trick to work.

I think within the confines of the question (notably - indices start at 1), that's not necessarily a problem.